As generally known, a wire cut electric discharge machine causes an electric discharge between a wire electrode and a workpiece when processing the workpiece for various kinds of machining processes, such as cutting, boring and shaping. Generally, the workpiece is held at ground potential and the wire electrode is held at a discharge potential, during electric discharge machining. Therefore, in most cases, the workpiece is connected electrically through the work table to a metallic bed.
Electric discharge machining entails a generation of heat in the machining section, and accordingly, a wire cut electric discharge machine for precision machining employs a heat nonconductive stone surface plate, such as a granite surface plate, as a horizontal slide table, and a work rest formed of a heat nonconductive material, to avoid deterioration of the machining accuracy due to a thermal deformation of the work table. The workpiece is connected electrically to the bed of the electric discharge machine by a grounding wire to hold the workpiece at ground potential.
Such a conventional electric discharge machine equipped with a work table formed of a heat nonconductive material, such as granite, having a comparatively small coefficient of linear expansion is able to avoid the deterioration of machining accuracy attributable to heat. However, grounding the workpiece by a grounding wire increases the reactance (L), entailing a reduction of machining speed and a deterioration of accuracy in detecting the progress of machining through a change of voltage.